The Sertoli cell provides the milieu in which the sequence of developmental events of the germ cells occur; no doubt the spermatogenic process is very largely dependent upon this ramifying cell type, yet evidence of the way in which its control over the germ cells is exerted remains elusive. In the past, little effort has been directed at this problem since most investigators were concerned primarily with the renewal and differentiation of the germ cells. The objective of this proposal is to examine the fine structural modifications of the Sertoli cells following a variety of experimental conditions such as hypophysectomy, hypophysectomy plus hormone replacement therapy, administration of exogeneous hormones to normal animals, experimental cryptorchidism, etc. The role of the Sertoli cell in establishing the blood-testis barrier will be examined in the primates while its modification and control after experimental treatment will be studied both in rodents and primates. The blood-testis barrier may have a significant effect upon access of blood-borne nutrients and gonadotropins to the germ cells. The role of the Sertoli cell as a phagocytic element will be investigated using electron microscopic radioautography. Perhaps with this multilateral approach to the Sertoli cells more information can be obtained concerning its overall role in the control of spermatogenesis.